1. Brief Description of the Invention
The present invention relates to a new class of structural materials, having the near-zero coefficient of thermal expansion (CTE) and high stiffness-to-weight ratio required for space-based Strategic Defence Initiative (SDI) systems. These structures have a CTE of less than 1 ppm/.degree.C., preferably less than 0.3 ppm/.degree.C., and have a specific stiffness more than twice that possible with aluminum.
In addition, they have high- and low-temperature capability (-50.degree. to 150.degree. C.), impact resistance, high structural damping, and survivability to a variety of mission-specific threats.
Metallic structures are heavy, and conventionally designed structures made from fiber-reinforced composites cannot meet all the demands placed on space-based systems without severe weight penalties, high cost, and slow fabrication. Microcomposite structures made from self-reinforced ordered polymer films, and possibly secondary materials, have excellent intrinsic and tailorable properties to fulfill ultralightweight SDI structural design requirements and the need for rapid fabrication.
Poly-p-phenylene benzobisthiazole (PBZT) and poly-p-phenylene benzobisoxazole (PBO) are liquid-crystalline ordered-polymer materials with specific strength and stiffness, CTE, and Other mechanical properties equal to or better than many fiber-reinforced composites. Ordered polymers in thin-film form (0.2 to 2 mils) become highly efficient microcomposites when impregnated with secondary material (epoxy, sol-gel, polyimide).
As described in Section 2, the excellent properties exhibited by thin-film PBZT can be translated into a new class of thin-walled structures which includes stiff, ultralightweight structures (ULWS) for space-based SDI systems. We have demonstrated that a PBZT honeycomb sandwich structure can achieve near-zero CTE ( at least as low as 0.3 ppm/.degree.C.) and high specific stiffness, with potential weight savings of 40 to 50 percent over graphite fiber composites, for applications requiring dimensional control of space-based SDI platforms.
The ULW sandwich structure with a specific shear stiffness of the core greater than 34.times.10.sup.6 in. (863.times.10.sup.3 m), about 30 percent better than aluminum, was designed and fabricated with thin (less than 0.002 in., 0.05 mm), biaxially-oriented PBZT film. In addition, near-zero CTE was achieved through a temperature range of -50.degree. C. to +150.degree. C. Tables 1--1 and 1-2 as well as FIG. 1 compare these properties to current state-of-the-art graphite fiber reinforced cores as well as commercially available cores. This particular PBZT structure has potential applications especially where minimum deflection and dimensional stability during thermal cycling in space is critical.
The advantage of using thin, high-modulus PBZT films is illustrated in FIG. 2 for several core materials. Current quasi-isotropic graphite-reinforced composite materials show high specific stiffness but are limited by their thickness. For the same shear modulus and core thickness, weight savings of nearly 38 percent can be achieved by using PBZT as the core material instead of the thickness-limited graphite/epoxy material. Further reduction in wall thickness is possible by reducing the amount of adhesives, for example, and should bring weight savings to 50 percent or more.
These structures have both extremely high flexural stiffness, at least 70,000 psi, combined with extremely low core density, no more than 5 pcf, preferably no more than 3 pcf. The high flexural stiffness arises from the inherent stiffness of the films employed, which can be made thin and accordingly are usable with a cell size of 1/8-inch or preferably less, as well as from the stiffness of the facesheets, which can be made from the same material as the core.
The weight savings realized by applying the ultralightweight PBZT core to SDI space-based applications allows significant performance improvements in sandwich structures. For example, thicker facesheets and larger facesheets separation distances are possible without any increase in weight over the baseline graphite honeycomb core sandwich to improve tracking accuracy and dimensional stability. Also, thin-films permit smaller cell sizes (less than 0.15 in.) without any increase in core weight. This reduces facesheet wrinkling and allows for thinner facesheets for antenna and mirror applications.
2. Background of Ordered-Polymer Films
This section will give an overview of the ordered-polymer films which are of great interest in connection with this invention. Current graphite/epoxy (Gr/Ep) or thermoplastic (TP) composites have high specific stiffness properties but cannot be processed economically as an ultrathin gage (less than 0.010 in., 0.25 mm) structural material. Thin-film PBZT can achieve the high specific stiffness required of SDI space-based designs as ultrathin, biaxially-oriented film. The ability to tailor properties such as stiffness, strength, CTE, and toughness are also important for optimizing such structural designs. Emphasis, then, is placed on applying thin-film PBZT to generic structures that require a maximum stiffness-to-weight ratio.
2.1 Development of Film-Based Ordered Polymers
The assignee of the present invention, Foster-Miller, Inc., has developed film-processing techniques for producing biaxially-oriented ordered polymer films from lyotropic solutions of high-viscosity aromatic heterocyclic polymers. See Ser. No. 07/098,710. FIGS. 3(a) and 3(b) show two such rigid-rod polymers, poly-p-phenylene benzobisthiazole (PBZT) and poly-p-phenylene benzobisoxazole (PBO). The presence of aromatic groups and ring-shaped stiffening elements (heterocyclic rings containing nitrogen and sulfur) in the backbone of the polymer molecule give rise to its excellent mechanical properties as well as high thermo-oxidative resistance. In addition to film processing, Foster-Miller has developed novel solution processing and treatment to control orientation, microfibrillar structure and texture, and physical properties. See Ser. Nos. 07/098,710 and 07/064,746.
Generally, the structural materials of the present invention contain polybenzazole (PBZ) polymers selected from the group consisting of polybenzoxazole (PBO), polybenzothiazole (PBT) and polybenzimidazole (PBI) polymers, and random, sequential or block copolymers thereof. Polybenzazole polymers and their synthesis are described at length in numerous references, such as Wolfe et al., Liquid Crystalline Polymer Compositions, Process and Products, U.S. Pat. No. 4,533,693 issued Aug. 6, 1985, and W.W. Adams et al., The Materials Science add Engineering of Rigid-Rod Polymers (Materials Research Society 1989), which are incorporated herein by reference.
Polybenzazole polymers preferably contain a plurality of met units that are AB-PBZ met units, as represented in Formula 1(a), and/or AA/BB-PBZ met units, as represented in Formula 1(b) ##STR1## wherein:
Each AR represents an aromatic group. The aromatic group may be heterocyclic, such as a pyridinylene group, but it is preferably carbocyclic. The aromatic group may be a fused or unfused polycyclic system. The aromatic group preferably contains no more than about three six-membered rings, more preferably contains no more than about two six-membered rings and most preferably consists essentially of a single six-membered ring. Examples of suitable aromatic groups include phenylene moieties, biphenylene moieties and bisphenylene ether moieties. Each Ar is most preferably a 1,2,4,5-phenylene moiety.
Each Z is independently an oxygen atom, a sulfur atom or a nitrogen atom bonded to an alkyl group or a hydrogen atom. Each Z is preferably oxygen or sulfur (the polymer is preferably PBO, PBT or a copolymer thereof);
Each DM is independently a bond or a divalent organic moiety that does not interfere with the synthesis, fabrication or use of the polymer. The divalent organic moiety may contain an aliphatic group (preferably C.sub.1 to C.sub.12), but the divalent organic moiety is preferably an aromatic group (At) as previously described. Each DM is preferably a 1,4-phenylene moiety or a 4,4'-biphenylene moiety, and is most preferably a 1,4-phenylene moiety.
The nitrogen atom and the Z moiety in each azole ring are bonded to adjacent carbon atoms in the aromatic group, such that a five-membered azole ring fused with the aromatic group is formed.
The azole rings in AA/BB-PBZ met units may be in cis- or trans-position with respect to each other, as illustrated in polybenzothiazoles and Polybenzoxazoles, 11 Ency. Poly. Sci. and Eng. 601, 602 (J. Wiley & Sons 1988), which is incorporated herein by reference.
The polybenzazole polymer may be rigid rod, semirigid rod or flexible coil. It is preferably rigid rod in the case of an AA/BB-PBZ polymer or semirigid in the case of an AB-PBZ polymer. It more preferably consists essentially of AA/BB-PBZ met units. Exemplary highly preferred met units are illustrated in Formulae 2 (a)-(e). ##STR2## The polybenzazole polymer most preferably consists essentially either of the met units illustrated in Formula 2(a) (cis-PBO) or of the met units illustrated in Formula 2(c) (trans-PBT),
Each polymer preferably contains on average at least about 25 mer units, more preferably at least about 50 mer units and most preferably at least about 100 mer units, The intrinsic viscosity of cis-PBO or trans-PBT in methanesulfonic acid at 25.degree. C. is preferably at least about 10 dL/g, more preferably at least about 20 dL/g and most preferably at least about 30 dL/g.
The examples herein relate mainly to PBZT structures, but PBO is also usable as it is often interchangeable with PBZT. PBO is commercially available from the Dow Chemical Company. U.S. Pat. Nos. 4,772,678; 4,703,103; 4,533,724; 4,533,692; 4,225,700; 4,131,748; and 4,108,835 are of interest in connection with the manufacture of PBO and PBZT.
Also usable are thermotropic polymers, including wholly aromatic polyesters such as Vectra (trademark), which is available from Hoechst-Celanese, and is a naphthalene-based aromatic copolyester; or Xydar (trademark), which is available from Amoco Performance Polymers, and is a bisphenol-based aromatic copolyester.
Biaxially-oriented PBZT films exhibit properties of high strength and stiffness (as high as 280 Ksi (1.72 GPa) tensile strength, and 25 Msi (172 GPa) tensile modulus). They have controllable CTE (from -10.degree. to +20.degree. ppm/.degree.C.), and outstanding thermal-oxidative resistance of over 400.degree. C. PBO is believed to have even higher specific modulus than PBZT, based on preliminary data from Dow Chemical on their PBO fibers. The-control over properties and characteristics of PBZT films as thin as 0.0015 in. (0.04 mm) is accomplished through engineering at a microcomposite level of about 10 to 100 times smaller than fibers or fabrics; resulting in high-performance, ultrathin structures. Orientation can vary from highly uniaxial (.+-.10 deg.) to quasi-isotropic (.+-.45 deg.), depending on the performance requirements of the SDI space-based structures. Table 2-1, listing the properties of PBO and PBZT film along with graphite/epoxy unidirectional composite, reflects the outstanding specific stiffness (modulus divided by density) of these ultrathin, ordered-polymer films.
The rod-like morphology of PBZT films is the key to both their excellent properties and their ability to be engineered into ultrathin composites and laminates. The rigid-rod molecules in the form of biaxially-extruded film consist of an interconnected microfibrillar network, similar to unidirectional plies stacked together as a single film, as shown schematically in FIGS. 4(a) and 4(b). The dimensions of this polymer network are on the order of 50 to 100 .ANG. in size, and during processing it can be infiltrated by a secondary material such as a dissolved or uncured polymer, or can be completely consolidated into 100% PBZT or PBO. The secondary or matrix material, if used, encapsulates the PBZT microfibrils and acts concert with the network to form a microcomposite having volume percentages of 5% to 70% secondary material, for example. See also Ser. No. 07/064,746, regarding these microcomposites. These microcomposites are known as "prepregs" in that the polymer network is pre-impregnated with the matrix material.
The properties of the PBZT microcomposite are tailorable, depending on the matrix material used. See Ser. No. 07/064,746. Efforts conducted at Foster-Miller have shown that a variety of matrix materials can form different types of microcomposites, including polyimide, epoxy and silica glass processed by the sol-gel method. The impregnation process can also be used to bond adjacent PBZT/matrix plies together to build up a laminate with quasi-isotropic properties. A thin coating of high-temperature resins, such as polyimide on the surface of the PBZT film, can serve as adherent for bonding PBZT film. This is useful for node bonds in honeycomb cores or joints on stiffened panels.
2.2 Application of Ordered Polymer Films to Ultralightweiqht Structures
One of the major advantages of using film-based PBZT for structural application is the film's ability to achieve high specific stiffness at a thickness of less than 0.002 in. (0.05 mm). Thin-walled structures such as tubes, ribbed panels and honeycomb sandwich structures fail at the elastic buckling stress of the structure (also called shell buckling). This buckling stress is directly related to the modulus and in general is much lower than the material's compressive stress. Thus, high-modulus, low-density thin materials such as PBZT films are required for space-based satellite structures.
When using an isotropically balanced eight-ply composite laminate for SDI structures, ultrathin plies are needed for high-stiffness, moderate-strength, minimum-gage applications, with a minimum gage being 0.040 in. (1.0 mm). However, future requirements for space structures will call for a gage of less than 0.010 in. (0.25 mm) for similarly balanced six- or eight-ply laminates (1). Production costs are extremely high for these fiber-reinforced sheets because it is difficult to spread and handle high modulus tows or yarns. PBZT films have the ability to exceed specific stiffness of 3.2.times.10.sup.8 in. as an ultrathin, quasi-isotropic material that is eight times thinner (less than 0.005 in., 0.127 mm) than current graphite-fiber prepregs. This will permit thin, dimensionally stable, high-stiffness structures to be fabricated.
High strength and high modulus carbon-fiber-reinforced composites (epoxy and thermoplastics) will play important roles in SDI space-based systems, but innovative new designs will also be needed. Some of these new structures are shown in FIGS. 5(a)-5(g).
Emphasis of SDI requirements for space-based systems has shifted from space-based interceptors (SBI) to space surveillance and tracking system (SSTS). These large, boxlike structures and platforms must be of low mass and high dimensional stability. New innovative structural designs can be realized only by using high modulus, thin-film PBZT as the primary structural material. Metals are too heavy and fiber composites are too thick.
In Section 3, we describe the design and fabrication of a thin-walled, ultralightweight honeycomb core using PBZT as the core material. The core provides high shear stiffness (98 Ksi at 5 pcf core density) and low CTE (0.3 ppm/.degree.C.) to meet the requirements of space surveillance and tracking systems. Furthermore, ULW PBZT honeycomb sandwich structures have the potential for producibility at an affordable rate, since films can be produced more rapidly than thin fiber-reinforced/epoxy prepregs.